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Class 8, Science, Chapter-10, Lecture-1, Sound Wave (Notes)

SOUND:

Sound is a form of energy produced due to the vibration of objects that causes a sensation of hearing in our ears.. 

Vibration: 

A kind of rapid to-and-fro motion of an object about a central position is called vibration or oscillation.
Example: a stretched rubber band when plucked vibrates and produces sound.

Activity to show that vibrating bodies produces sound:

Strike the prongs of a tuning fork on a rubber pad and bring it near the ear. We can hear a sound. 
If a suspended table tennis ball is touched with the vibrating prong, the ball is pushed away repeatedly. 
This shows that the prong is vibrating, and vibrating objects produces sound.

Examples of how certain sounds are produced:

In flute, the air column vibrates to produce sound. 
In stringed musical instruments like sitar, veena etc, there is a tightly stretched string which vibrates to produce sound. In humans, sound is produced by the voice box or the Larynx. Vocal cords in the larynx vibrate and produce sound. 
When a bird flaps its wings, a sound is produced.

Activity to show that sound needs a material medium for its propagation: 

Suspend an electric bell in an airtight bell jar. Connect the bell jar to a vacuum pump. If the switch is pressed, we can hear the sound of the bell. 
If air is pumped out through the vacuum pump, we cannot hear the sound of the bell. 
This shows that sound needs a medium to travel and sound cannot travel in vacuum

Propagation of Sound: 

The particles of sound wave do not move from one place to another. 
They just oscillate back & forth about their position of rest. 
The disturbance is carried forward in the direction parallel to the direction of oscillation.

CHARACTERISTICS OF WAVE:

  1. Amplitude 
    The maximum displacement of a vibrating particle from its central position is termed as the amplitude of the 
    It is denoted by A. Its SI unit is metre (m)
  2. Time Period
    The time taken by a vibrating object to complete one oscillation is called time period of the vibration. 
    It is denoted by T. Its SI unit is second (s)
  3. Frequency
    The number of oscillations completed per unit time is called frequency of the wave. 
    It is denoted by f. Its SI unit is hertz (Hz)
  4. Wave Speed
    The distance travelled by a wave per unit time is called wave speed. 
    It is denoted by v. Its SI unit is metre per second (ms-1).

Relation Between Time Period and Frequency: 

Let T seconds be the time period of a wave and f be the frequency. 
(Number of waves produced in 1 s)= 1

(Number of waves produced in 1 s)= ${1 \over {\rm{T}}}$

$ \Rightarrow $     ${{\rm{  f   }} = {\rm{   }}{1 \over {\rm{T}}}{\rm{ }}}$ AND ${{\rm{  T   }} = {\rm{   }}{1 \over {\rm{f}}}{\rm{ }}}$

FACTORS AFFECTING THE SPEED OF SOUND:

  1. Nature of the medium: 
    Speed of sound depends on the nature of the medium(material) through which it travels. 
    Speed of sound is different in different mediums 
    The speed of sound is more in solids, less in liquids and least in gases.
  2. Temperature of medium: 
    The speed of sound also depends on the temperature of the medium. 
    If the temperature of the medium is more, the speed of sound is more
  3. Humidity of medium: 
    Speed of sound increases with increase in humidity.